Turbine blade with a reduced mass

ABSTRACT

The invention relates to a turbine comprising at least four stages and to the use of a rotor blade with a reduced mass. In prior art, rotor blades in the fourth stage of a gas turbine, which exceed 50 cm in length, cause problems relating to mechanical strength, as centrifugal forces of too great a magnitude occur during the rotation of the rotor blades. An inventive rotor blade in the fourth row of a gas turbine has a reduced density as a result of a high proportion of a ceramic, thus reducing the centrifugal force.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP02/14499, filed Dec. 18, 2002 and claims the benefit thereof.The International Application claims the benefits of European Patentapplication No. 02001348.8 EP filed Jan. 18, 2002, both of theapplications are incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a turbine with at least four stages inaccordance with claim 1 and to the use of a turbine blade with a reduceddensity in accordance with claim 9.

The use of ceramic guide vanes in gas turbines is known from U.S. Pat.No. 3,992,127. Ceramic guide vanes are used because the ceramic has goodhigh-temperature properties. Particularly high temperatures, which onlyceramics are able to withstand, occur in particular in the first rowdownstream of the combustion chamber (first turbine stage), with theturbine blades and vanes in the first row being the smallest.

U.S. Pat. No. 5,743,713 has disclosed a ceramic blade which is insertedinto a metallic rotor disk of a turbine.

U.S. Pat. No. 4,563,128 has disclosed a ceramic blade which has ametallic core surrounded on the outside by ceramic and extending as faras a radial end of the blade. The metallic core forms a very highproportion of the volume.

Hitherto, ceramic rotor blades have only been used, by virtue of theirhigh thermal stability, in the temperature-critical stage or stages of aturbine, whereas in the subsequent stages it has been customary to usemetallic rotor blades (in particular made from Ni-based alloys or fromTiAl alloys).

A significant improvement to the efficiency of gas turbines can beachieved if, at least from the fourth stage onward, the turbine rotorblades are increased in size by, for example, approximately 20% comparedto conventional dimensions. This increase in size from the fourth stageonward, however, leads to a considerable increase in the centrifugalforces at the blades if the rotational speed remains unchanged, andthese forces represent unacceptable loads on these blades and on thedisks to which the blades are secured.

SUMMARY OF THE INVENTION

Therefore, it an object of the invention to provide a turbine with anincreased efficiency compared to a turbine with conventional blading.

The object is achieved by virtue of the fact that the turbine, in thefourth stage, in each case has rotor blades with a length of at least 50cm which contain a high proportion of a material with a density of atmost 4 g/cm³, and are, for example, made from ceramic, with the resultthat the mass is significantly reduced compared to standard metallicblading of conventional dimensions. This allows the blade length, or atleast the length of the main blade section, to be lengthenedconsiderably compared to metallic blades.

It is even possible to use solid-ceramic or hollow-ceramic blades whichare secured to metallic disks of the turbine rotor, as is known fromU.S. Pat. No. 5,743,713.

It is also advantageous to use ceramic rotor blades which have ametallic core which is surrounded by ceramic. In this case, theproportion by volume of the ceramic is very high, so that the mass isgreatly reduced compared to a purely metallic blade with an optionalthin ceramic protective layer.

A further advantage of a more lightweight blade is that the mechanicalloading on the disk to which the blade is secured is lower duringrotation on account of the lower mass attached thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures diagrammatically depict the invention, which is explained inmore detail below with further details and advantageous refinements.

In the drawing:

FIG. 1 shows a gas turbine,

FIG. 2 shows a partial region of a gas turbine with a fourth rotor bladestage,

FIG. 3 shows a rotor blade and a rotor disk,

FIG. 4 shows a section on line IV-IV in FIG. 3,

FIG. 5 a, b show further exemplary embodiments of a rotor blade.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 diagrammatically depicts a longitudinal section through aturbine, for example a gas turbine 41. However, the invention is notrestricted to a gas turbine.

A compressor 47, a combustion chamber 50 and a turbine part 53 arearranged in succession along a turbine shaft which includes a tie rod 4.The turbine part 53 has a hot-gas duct 56. Gas turbine blades and vanes13, 16 are arranged in the hot-gas duct 56. Rings of guide vanes andrings of rotor blades are provided alternately. The gas turbine bladesand vanes 13, 16 are cooled, for example, by combined air and/or steamcooling. For this purpose, by way of example, compressor air is removedfrom the compressor 47 and fed to the gas turbine blades and vanes 13,16 via an air passage 63. Steam is also fed to the gas turbine bladesand vane 13, 16 via a steam feed 66, for example. This steam preferablyoriginates from a steam turbine of a combined-cycle gas and steamprocess.

FIG. 2 shows an excerpt from a gas turbine 41. The gas turbine 41 has aturbine shaft with a tie rod 4 which rotates about an axis 7. Aplurality of guide vanes 13 and a plurality of rotor blades 16, whichare arranged, for example, in the hot-gas duct 56, extend in the radialdirection 19, which runs perpendicular to the axis 7. There are at leastfour rows of rotor blades and, for example, four rows of guide vanes,i.e. there is a total of four stages. The first row of guide vanes may,for example, may be replaced by a special burner arrangement. Just oneof the blades 16 in the fourth stage is illustrated here, by way ofexample.

The rotor blades 16 are, for example, secured to metal disks (25, FIG.3) on the turbine shaft, held together by the tie rod 4, and rotate withthe tie rod 4 about the axis 7.

The guide vanes 13 are secured in a rotationally fixed position to acasing 10 of the gas turbine 41.

A hot gas 22 flows in the direction of the axis 7, from the left to theright in the drawing, as is diagrammatically indicated by an arrow.

The fourth row of rotor blades, as seen in the direction of flow 22, isdenoted by V4. The rotor blades in the fourth stage are in each caserotor blades 16 which have a high proportion by volume of their materialmade up of a material with a density of at most 4 g/cm³ and are made,for example, from ceramic and have a length of at least 50 cm, inparticular of at least 65 cm.

Since the density of ceramic materials is in the range from 1.5 to 3.5g/cm³, and is therefore well below the densities of nickel-base alloys,at 8 g/cm³, and of TiAl alloys, at approximately 4.5 g/cm³, a ceramicrotor blade of this type has a considerable reduction in mass comparedto a corresponding metallic rotor blade, so that, when these rotorblades are rotating, lower centrifugal forces occur, in particular atthe outer radial end 37 of the rotor blade 16, thereby inducing loadingin particular on the root of the rotor blade 16 and its anchoring in theturbine shaft.

By lengthening the turbine rotor blades in the fourth row by, forexample, approximately 20% , it is possible to considerably increase theefficiency of gas turbines. Ceramic rotor blades are, for example, madecompletely from ceramic, in which case the ceramic may advantageouslycomprise various layers of ceramics. For example, it is possible to usefiber-reinforced CMC oxide ceramics or fiber-reinforced CMC nonoxideceramics, nonoxidic ceramics, such as for example carbon fibers or SiCfibers in a corresponding carbon or silicon carbide matrix. It is alsopossible to use oxide systems, e.g. mullite fibers or aluminum oxidefibers in a mullite matrix.

The ceramics may in turn be coated with a protective layer 36 (FIG. 4 a)to prevent corrosion and oxidation, such as those which are known frommetallic turbine blades: yttrium-stabilized zirconia, boron nitride,spinels.

FIG. 3 shows a rotor blade 16 with a length L between platform 17 andradial end of the rotor blade 16 which is formed, for example, entirelyfrom ceramic and is inserted into a metallic rotor disk 25 in a mannerfixed in terms of rotation. The metallic disk 25 is connected to the tierod 4 and rotates therewith.

The diameter of the disk 25 is no greater than usual and is also notexposed to the highest temperatures within the hot-gas duct 56, andconsequently metal can continue to be used as material for the disk 25,in the same way as in a conventional turbine.

It is also possible to use what are known as hybrid turbine blades,which still have a metallic core but this core is surrounded by aceramic, as is known, for example, from U.S. Pat. No. 4,563,128. Thecontent of disclosure of this document relating to the structure of theceramic turbine blade is expressly incorporated in the content ofdisclosure of the present application. Further types of hybrid bladesare conceivable.

FIG. 4 shows an example of a hybrid blade 16. A main blade section 28 atits outer surface consists of ceramic 39. In the interior, there is ametallic core 31, for example formed from a nickel and/or cobaltsuperalloy. The metallic core 31, by way of example, also forms a rootpart 34 of the blade 16.

In the radial direction 19, the metallic core 31 does not extend all theway to the radial end 37 of the blade 16, but rather, for example, onlyextends over for example 70% of the length of the main blade section 28in the radial direction 19, since otherwise the loads caused by thecentrifugal forces at the intended rotational speed of the turbine wouldexceed the mechanical strength of the metallic core or of the blade rootor of the anchoring in the turbine shaft.

The metallic core 31 may at least in part be formed from metallic foam,in order to save further weight.

The proportion by volume of the material formed by the ceramic isamounts to least 40% or, for example, even exceeds that of the metalliccore 31, so that the blade 16 has a high proportion by volume of itsmaterial formed by ceramic.

The proportion of ceramic 39 may also be located predominantly at theend 37 of the blade 16, since that is where the centrifugal forces arehighest (FIG. 5 a).

A remaining part 38 of the blade 16 consists of metal, for example of anickel and/or cobalt superalloy. The hybrid blade 16 may also be ofinternally hollow design, in order to further reduce its weight.

It is also possible, as illustrated in FIG. 5 b, to provide a skeleton40 made from metal, for example from a nickel and/or cobalt superalloy,into which ceramic parts are introduced.

The skeleton 40 comprises, for example, a leading edge 70, which themedium strikes first in the direction of flow, a trailing edge 73, theroot part 34 and the tip 76, as well as the radial end 37.

The rotor blade 16 may also be internally hollow and cooled by airand/or steam cooling with or without film-cooling bores.

It has not hitherto been known that ceramic rotor blades with a lengththat is considerably increased compared to conventional dimensioning, onaccount of their lower density and the associated reduction in thecentrifugal forces, can advantageously be used to increase the turbineefficiency.

1. A gas turbine blade for a fourth stage and onward of a multi-stageturbine, the blade comprising: a metallic root portion; a platformportion; and an airfoil portion comprising at least a structural ceramicmaterial for bearing a tensile load to oppose a centrifugal force thatdevelops during rotation of the blade, wherein the root, platform andairfoil are collectively comprised of a plurality of materials in whichat least 40% by volume of the materials comprise the structural ceramicmaterial having a density of at most 4 g/cm³, wherein the density byvolume provided by the plurality of materials allows providing a lengthof at least 50 cm for a blade disposed in the fourth stage and onward ofthe multi-stage turbine.
 2. The turbine blade as claimed in claim 1,wherein the turbine blade is arranged in a metallic rotor disk.
 3. Theturbine blade as claimed in claim 1, wherein the turbine blade has astructural metallic core surrounded by a structural ceramic material. 4.The turbine blade as claimed in claim 3, wherein the metallic core isformed at least in part from a metallic foam.
 5. The turbine blade asclaimed in claim 1, wherein the ceramic material has a non structuralceramic protective layer arranged over the ceramic material.
 6. Theturbine blade as claimed in claim 2, wherein the length of the turbineblade is at least 65 cm.
 7. The turbine blade as claimed in claim 1,wherein the turbine blade has a metallic skeleton material thatfunctions as a structural frame and is adapted to support a structuralceramic material.
 8. The turbine blade as claimed in claim 1, whereinthe materials are a ceramic material or a glass material.
 9. The turbineblade as claimed in claim 1, wherein the material with the density of atmost 4 g/cm³ is a carbon-containing material.
 10. A turbine blade for afourth stage and onward of a multi-stage turbine, the blade comprising:a root portion connected to a rotor disk; an airfoil having a firstsection located adjacent to the root portion, wherein the first sectioncomprises a material having a first density, the airfoil having a secondsection located adjacent to the first section consisting exclusively ofan structural ceramic material having a second density different thanthe first density and extending at least 80% of the length of the tipportion, wherein the structural ceramic material bears a tensile load tooppose a centrifugal force that develops during rotation of the blade,wherein at least 40% by volume of the first and second sections comprisethe structural ceramic material having a density of at most 4 g/cm³,wherein the density by volume achieved over the first and secondsections of the airfoil allows providing a length of at least 50 cm fora blade disposed in the fourth stage and onward of the multi-stageturbine.
 11. A gas turbine blade for a fourth stage and onward of amulti-stage turbine, the blade comprised of at least one material inwhich at least 40% by volume of the material has a density of at most 4g/cm³, wherein the density by volume achieved by the at least onematerial allows providing a length of at least 50 cm for a bladedisposed in the fourth stage and onward of the multi-stage turbine,wherein the at least one material bears a tensile load to oppose acentrifugal force that develops during rotation of the blade.
 12. Theturbine blade as claimed in claim 11, wherein the turbine blade has ametallic skeleton into which ceramic parts are introduced.
 13. Theturbine blade as claimed in claim 11, wherein the material with thedensity of at most 4 g/cm³ is a ceramic material or a glass material.14. The turbine blade as claimed in claim 11, wherein the material withthe density of at most 4 g/cm³ is a carbon-containing material.
 15. Theturbine blade as claimed in claim 11, wherein the turbine blade has ametallic core surrounded by a ceramic material, the metallic core andceramic material both adapted to provide structural support.
 16. Theturbine blade as claimed in claim 15, wherein the metallic core isformed at least in part from a metallic foam.
 17. The turbine blade asclaimed in claim 11, wherein the ceramic material has a protectivelayer.
 18. A gas turbine blade for a fourth stage and onward of amulti-stage turbine, the blade comprising: a metallic root; and aplatform comprising a structural ceramic material mechanicallyinterlocked with the root, the platform ceramic material extendingradially to form an airfoil, wherein the ceramic material bears atensile load to oppose a centrifugal force that develops during rotationof the blade.
 19. The turbine blade of claim 18 wherein the metallicroot comprises one or more affixing ribs at a first portion incorrespondence with the platform for establishing the mechanicalinterlocking with the platform.
 20. The turbine blade of claim 19wherein the metallic root further comprises a second portion extendingradially through a portion of the airfoil.
 21. The turbine blade ofclaim 20 wherein the structural ceramic material comprises a volume ofat least 40% of the airfoil volume, including the metallic root secondportion therein, thereby reducing blade weight to provide a length of atleast 50 cm for a blade disposed in the fourth stage and onward of themulti-stage turbine.
 22. A gas turbine comprising at least four stagesof successively arranged turbine blades and vanes, wherein each stagecomprises a row of rotor blades and a row of guide vanes, with the rotorblades having a metallic root part, wherein at least the fourth row ofrotor blades comprises rotor blades in which at least 40% by volume ofthe material has a density of at most 4 g/cm³, so that the mass issubstantially reduced compared to a metallic rotor blade, wherein aminimum length of the rotor blades is 50 cm and further wherein at leastbeyond 80% of the length of a main blade section in a radial directionconsists exclusively of ceramic.